This invention relates to a level meter that employs the radar principle for measuring the fill level of a medium in a container and features an electric conductor system for conducting an electromagnetic signal into the container and returning reflected components of the electromagnetic signal from the container, said electric conductor system terminating at a predefined level in the container. A level meter of this type is described in DE 102 40 550 A1.
The measuring process in a level meter that employs radar technology and features an electric conductor system for conducting an electromagnetic signal into and back from a container is based on the time domain reflectometry (TDR) principle that has been used, for instance, in cable testing and resembles the mode of operation of conventional radars. For example, a TDR level meter of this type transmits an extremely short electric pulse via an essentially straight electric conductor into a container holding a medium such as a liquid, powder or granular substance whose fill level is to be determined. The electric conductor system is typically designed as a single or dual conductor that extends into the medium. If the electric conductor system is composed of dual conductors, these may be, for instance, in the form of two parallel lines or of a coaxial cable.
In the case of a dual-conductor configuration, an electric pulse fed into it practically travels “between” the two conductors into the container, where it is at least partially reflected off the surface of the medium and the reflected component of the short electric pulse is typically received by a processing electronics unit that measures the signal runtime and thus the level of the medium in the container. The reflected component of the short electric pulse varies as a function of the dielectric constant or specific inductance capacity of the medium and increases as the latter increases. The signal runtime is proportional to the level of the medium in the container. Varying environmental conditions, such as increasing or decreasing atmospheric pressures or ambient temperatures do not negatively affect the measuring accuracy of the TDR level meter. Moreover, the run-time of the signal is unaffected by the dielectric constant of the medium whose level is to be measured.
Conventional level meters employing the radar principle and featuring an electric conductor system of the type described above have so-called “dead zones”, which are fill-level regions of the medium that either cannot be measured, or are measured in a highly imprecise fashion at best. Due to physical discontinuities in the area of the conductor system around the immersion point in the container, there will generally be an “upper dead zone”, with a “lower dead zone” at the end of the conductor system closest to the bottom of the container. These dead zones generate relatively strong reflected components of the electromagnetic signal which tend to be significantly superimposed on the signal component reflected by the medium in these regions. The existence of such dead zones is generally viewed as a drawback compromising the level measurements made by means of the level meter.